Solar thermal power plants represent an alternative to conventional power generation from water power, wind power, nuclear power or fossil fuels. They make use of solar radiation energy to produce electrical energy and are embodied as solar thermal subassemblies with, for example, parabolic trough collectors, Fresnel collectors or solar tower receivers.
Such power plants typically comprise either a singular working fluid circuit system, in which a working fluid is directly evaporated in this circuit, or a first solar power plant section for the absorption of solar energy and a second, mostly conventional power plant section with a steam turbine system powered by a working fluid. This second power plant type with two separated circuit systems is referred to as a solar thermal power plant using indirect evaporation, since the solar energy is only employed here indirectly to evaporate the working fluid.
A conventional solar thermal power plant using indirect evaporation typically comprises a solar thermal subassembly such as, for example, a solar array made from parabolic trough collectors, Fresnel collectors or a tower receiver, wherein a heat transfer medium is heated, a heat exchanger group wherein the heat energy is transferred from the heat transfer medium to a working fluid in a steam circuit such as, for example, a water steam circuit, and optionally from a thermal storage tank. In the heat exchanger group, the heat energy is typically released in three stages from the primary circuit into the working fluid in the steam circuit, comprising preheating, evaporation and superheating. In solar thermal power plants using indirect evaporation, heat transfer oils, water, air or molten salts have been used up until now as the heat transfer medium in the primary circuit, wherein water is generally used as the working fluid of the secondary circuit in the steam circuit.
At present, parabolic trough collectors with heat transfer oil as a heat transfer medium, Fresnel collector solar arrays with water as a heat transfer medium and tower receiver power plants with molten salts, air or water as a heat transfer medium comprise the majority of these power plant types in use. Temperatures of up to 390° C. for heat transfer oil plants and temperatures of up to 550° C. for molten salts can be feasibly attained, though temperatures of up to 1100° C. can be attained with tower receivers using air as the heat transfer medium.
Superheating the working fluid in the steam circuit serves to increase the efficiency of the plant in solar thermal power plants; it is employed when operating steam turbines to prevent the turbine blades from being damaged by condensed drops of liquid which remain in the steam after the steam generation stage. At present, a steam turbine system comprising several steam turbines is often operated to improve the energy use of the hot heat transfer medium produced in solar thermal power plants. Here, one, two or more resuperheating processes are incorporated into the steam conduction system between the steam turbines, in particular in parabolic trough collector power plants using heat transfer oil as a heat transfer medium. During this resuperheating process, live steam generated in the superheating stage is conducted via a high-pressure turbine. Before entering the downstream (in terms of the steam direction) process unit, such as a mid-pressure or low-pressure turbine, the steam is then conducted into a resuperheater with one, two or more heat exchangers, where it is once again superheated by the hot heat transfer medium diverted before the superheating stage.
The presently most common method of connecting heat transfer media in a solar thermal power plant using indirect evaporation and single resuperheat is represented in FIG. 1. Here, the oil used as a heat transfer medium in the heat exchanger group is, on leaving the solar array, conducted first via the superheater, then via the evaporator and finally via the preheater of the secondary circuit. In addition, part of the hot oil is diverted before the superheater and is conducted via a resuperheating stage.
A further possibility for connection is to carry out the first resuperheating stage in two devices, or to superheat the steam a second time in a second resuperheating stage. Potential connection variants for solar thermal power plants using indirect evaporation and single or dual resuperheat are, for example, summarized in the Siemens patent applications WO 2009034577 A2 and WO 2010054911. All connection variants described therein are accepted with reference to this patent specification as the basic connection variants for the present invention.